Information for multiple users may be multiplexed in an orthogonal or non-orthogonal mode. Non-orthogonal multiplexing allows information for multiple users be transmitted on a same time-frequency resource.
With non-orthogonal multiplexing, code superposition may be applied at a transmitter, and Successive Interference Cancellation (SIC) may be applied at a receiver. For example, in downlink transmission, a transmitter may acquire a modulated symbol of a user 1 and a modulated symbol of a user 2 by respectively coding and modulating a stream of bits for the user 1 and a stream of bits for the user 2. The transmitter may acquire a superposed symbol by superposing one of the two modulated symbols on the other, and then send the superposed symbol on the same time-frequency resource. Superposition of the modulated symbols of the two users may be referred to as code superposition. With SIC, an interfering signal may be demodulated first. A signal of the user 1 may be referred to as the interfering signal, as it interferes with a signal of the user 2 that is to be demodulated. The demodulated interfering signal may be subtracted from a signal received by the user 2. The signal of the user 2 may then be demodulated.
Hierarchical modulation may be regarded as a variation of code superposition, where a constellation of a nature of Gray mapping may be acquired by mapping a combination of a stream of bits of a high priority and a stream of bits of a low priority onto a diagram of constellations. By Gray mapping, it means that combined bits mapped to adjacent constellation points on the diagram of constellations differ by but one bit. Combination of hierarchical modulation and SIC may achieve performance approaching a capacity limit. However, with hierarchical modulation, power is to be allocated separately for different streams of data, which is inflexible. It is complicated to implement capacity of a downlink channel for multiple users.